1. Field of the Invention
The present invention relates to a manufacturing method for an optical integrated circuit for use in optical communication or the like, and more particularly to a manufacturing method for an optical integrated circuit realizing the optical coupling between an optical waveguide and an optical active element by a spatial reflection type structure.
2. Description of the Related Art
With the recent explosive expansion of communication demands, developments on communication networks for higher-speed communication using an optical fiber and for higher-capacity communication using wavelength division multiplexing (WDM) are being pursued at a worldwide level. Many of optical functional elements required for repeating, switching, etc. in such WDM optical communication are configured as bulk components or the like, and a higher level of optical path length adjustment or the like is required with an increase in operating speed of an optical communication system. Accordingly, the components become expensive in many cases, causing a great increase in cost of the optical communication system.
In such a background, a planar lightwave circuit (PLC) type functional integrated element allowing finer optical path length design and mass production using a full process is expected as a promising device for realization of high functionality, size reduction, and cost reduction, and active research on such a device is being pursued globally at present. Further, it has become greatly important to develop a structure and its manufacturing technique allowing the control of such a PLC type WDM device and the coupling between the WDM device and an optical active element required for extraction of an optical signal from the WDM device or launching of an optical signal into the WDM device, with ease at a high density and a low cost.
As a coupling structure between the optical waveguide in the PLC type device and the optical active element, various simple optical mounting structures using a mirror having a surface inclined about 45° with respect to the optical waveguide have been proposed. There will now be described in detail various forming methods for such an inclined mirror used for the coupling between the optical waveguide and the optical active element. A forming method by a hybrid inclined mirror structure (which will be hereinafter referred to as a conventional forming method A) is known, wherein an inclined mirror is fabricated as a separate component by using anisotropic etching of Si crystal or the like, and this inclined mirror component is hybrid-integrated with the optical waveguide. This conventional forming method A is a method having been generally used until other methods to be hereinafter described are proposed.
A forming method by inclined dicing (which will be hereinafter referred to as a conventional forming method B) is known, wherein a wafer is diced in its inclined condition by using a dicing blade having a V-shaped cutting edge with respect to a substrate having an optical waveguide to form an inclined surface as one of the opposed diced faces of the wafer and a perpendicular end surface as the other diced face of the wafer. This conventional forming method B is an easiest process and has a merit in the case that the kind of products is limited.
A forming method by laser beam machining (which will be hereinafter referred to as a conventional forming method C) is known from U.S. Pat. No. 5,894,538, for example, wherein a laser beam is directed to an optical waveguide in a direction inclined 45° with respect to the optical waveguide, thereby vaporizing SiO2, or the amount of vaporization of SiO2 is controlled with the irradiation time by laser sweeping to perform 45° etching to the optical waveguide. This conventional forming method C does not require mask formation on a wafer and can change a forming position of the inclined surface according to wafers, so that this method has a merit in the case of flexible production such that the kind of products is various and the volume of products is low.
A forming method by different-rate wet etching (which will be hereinafter referred to as a conventional forming method D) is known from U.S. Pat. No. 5,135,605 and Japanese Patent Laid-open No. Hei 11-326662, for example, wherein a material having an etching rate higher than the material of an optical waveguide is deposited by evaporation on an inclined surface forming region of the optical waveguide, and an etching mask is formed over the optical waveguide to perform wet etching. In this conventional forming method D, the underetch under the etching mask proceeds faster in the region where the above high-etch-rate material is deposited than in the other region, so that asymmetrical inclined surface formation (e.g., formation of a perpendicular end surface and an incline surface) is allowed. The conventional forming method D is a full process per wafer, and it therefore has a merit in the case of mass production such that the kind of products is few and the volume of products is high.
A forming method by inclined RIE (Reactive Ion Etching) (which will be hereinafter referred to as a conventional forming method E) is known from Japanese Patent Laid-open Nos. 2000-199827 and 2000-235128, for example, wherein patterning is performed by using a thin mask film such as a metal mask, and a substrate to be etched is inclined about 45° (a desired angle of an inclined surface) with respect to an RIE electrode to perform RIE. This conventional forming method E adopts RIE, and it therefore has a merit from the viewpoint of smoothness of the inclined surface.
A forming method by a gray mask and RIE (which will be hereinafter referred to as a conventional forming method F) is known from U.S. Pat. No. 5,310,623 and Japanese Patent Laid-open No. Hei 6-265738, for example, wherein a gray-scale mask such that the exposure intensity is approximated by an opening area of a subpixel size is used to form a master pattern of an inclined surface on a photosensitive mask material by one operation of exposure and development, and the master pattern of the inclined surface is next transferred by RIE. In this conventional forming method F, the mask surface of the photosensitive mask material is heated during the RIE to cause a deterioration in mask shape in an RIE apparatus. Therefore, in the process of transferring the master pattern formed of the photosensitive mask material by the RIE, the mask material after exposure and development must be heated to a temperature higher than the heating temperature in the RIE apparatus for the purpose of curing, thereby preventing softening of the mask material during the RIE to prevent thermal deformation of the mask shape upon reheating.
In the conventional forming method F, the master pattern of a three-dimensional shape (e.g., inclined surface) formed of a photoresist for use in the transfer process by RIE is formed in one operation of exposure using a single exposure mask by approximating the three-dimensional shape according to the average of exposure intensities at a subpixel size forming a gray scale. Accordingly, as compared with the case of forming a three-dimensional mater pattern by multistep exposure using a plurality of exposure masks, this method F has a merit from the viewpoint of prevention of deterioration in shape of the master pattern due to misregistration of the exposure masks.
The coupling structure using the inclined mirror between the optical waveguide in the PLC type device and the optical active element may be generally classified into two structures shown in FIGS. 1 and 2. The structure shown in FIG. 1 is of a waveguide end surface reflection type, which is known from U.S. Pat. No. 5,966,478, for example, wherein an end surface of an optical waveguide is inclined about 45° with respect to the optical waveguide, and light propagating along the optical waveguide is totally reflected on the inclined end surface of the optical waveguide. To effect the total reflection on the inclined end surface of the optical waveguide, a material having a refractive index lower than that of the optical waveguide must be applied near the inclined end surface, or a metal film must be formed on the inclined end surface.
On the other hand, the structure shown in FIG. 2 is of a spatial reflection type such that light emerged from an end surface of an optical waveguide is totally reflected on an inclined surface inclined about 45° with respect to the optical waveguide and opposed to the end surface of the optical waveguide. In selecting the structures shown in FIGS. 1 and 2, it is necessary to consider a sealing structure for the optical active element. That is, the optical active element to be mounted on an optical integrated circuit must have a sealing structure for protection from moisture or the like. Conventionally, the optical active element is hermetically packaged and the optical coupling between the optical active element and an external element (e.g., optical fiber) is effected by utilizing a lens or the like through a glass window. Similarly, it may be considered to protect the whole of the optical integrated circuit on which the optical active element is mounted by using a hermetic package.
Basically, there is no level difference between the structures shown in FIGS. 1 and 2. However, when the optical integrated circuit becomes large in scale, the number of connection points to optical fibers or the like increases, so that the structure for optical coupling between the optical integrated circuit and the outside of the package is complicated to cause an increase in cost. Accordingly, the protective method using the hermetic package sealing the whole of the optical integrated circuit impairs the merit of the PLC type optical integrated circuit. The large-scale integration of the optical integrated circuit can be supported by sealing the optical active element with resin or the like locally applied to the periphery of the optical active element and adopting a simple connecting method such as butt joint for optical connection points.
In the case of applying this method to the structure shown in FIG. 1, there is a problem that the sealing material such as resin may flow down along the inclined surface. In general, the sealing material such as resin has a refractive index higher than that of air. As a result, the reflectance on the inclined surface is greatly reduced by the resin flowed down along the inclined surface, so it is therefore necessary to coat the inclined surface with metal or the like. In the case of coating the inclined surface, the inclined surface must be formed at a position exposed to the outside of the optical integrated circuit as at the end thereof. Accordingly, there is a substantial limit to the forming position of the inclined mirror, causing a great disadvantage in designing the optical integrated circuit.
In the spatial reflection type structure shown in FIG. 2, a transparent resin or the like can be easily applied before mounting the optical active element, so that the above limit to the designing of the optical integrated circuit can be eliminated. Regarding a forming method for the above-mentioned spatial reflection type structure that may be advantageous in the case of considering the sealing structure for the whole of the optical integrated circuit, the following subjects are assumed. The subjects to be described later are required to satisfy the practicable performance, reliability, compactness, and cost effectiveness of the inclined mirror, and they are indispensable for the future evolution of optical active integration in the optical integrated circuit.
The first subject is a subject on a monolithic structure between the perpendicular end surface of the optical waveguide and the inclined mirror. The first-subject contributes to the reliability of the mirror structure. For example, in the case of forming the spatial reflection type structure by using separate components, there is a possibility that an adhesive material for the separate components may expand because of variations in operation temperature conditions or the like, causing a degradation in optical coupling. Such a degradation in optical coupling can be avoided by forming a monolithic structure between the perpendicular end surface and the inclined mirror.
The second object is a subject on simultaneous formation of the perpendicular end surface of the optical waveguide and the inclined mirror. In general, it is difficult to form a mask for reprocessing a wafer near a deep groove formed on the wafer for forming the inclined surface. Accordingly, it is necessary to realize a forming method by which both the formation of the perpendicular end surface and the formation of the inclined surface can be performed simultaneously.
The third subject is a subject on the perpendicularity of the light emergent end surface to the optical waveguide (which will be hereinafter referred to as end surface perpendicularity). If the end surface perpendicularity is not satisfactory, the emergent light is refracted at the end surface to cause a limit to the coupling tolerance between the end surface and the optical active element (the deviation of the mount position of the optical active element allowing a required coupling efficiency). Therefore, it is desirable to realize a forming method which can sufficiently ensure the end surface perpendicularity.
The fourth subject is a subject on the distance between the perpendicular end surface of the optical waveguide and the inclined mirror (which will be hereinafter referred to as end surface-to-inclined surface distance). If the end surface-to-inclined surface distance is large, the reflection point of a light beam on the inclined surface is largely deviated by the refraction at the light emergent end surface, causing a limit to the coupling tolerance between the end surface and the optical active element. Therefore, it is necessary to realize a forming method which can reduce the end surface-to-inclined surface distance.
The fifth subject is a subject on the smoothness of the perpendicular end surface of the optical waveguide and the reflection surface of the inclined mirror. If the smoothness of the perpendicular end surface and the reflection surface is low, a reduction in optical coupling quantity due to scattering at each surface is invited. Therefore, it is desirable to realize a forming method which can ensure sufficient smoothness.
The sixth subject is a subject on localization of the inclined mirror forming region. By localizing the inclined mirror forming region, in-line optical coupling in a multichannel optical waveguide can be obtained to increase the degree of freedom of design of an optical waveguide pattern.
In relation to the above-mentioned subjects, the conventional forming methods A to F have the following problems. The conventional forming method A does not satisfy the first subject. That is, the hybrid inclined mirror structure by this method is not a monolithic inclined structure, because the inclined mirror is fabricated as a separate component by using anisotropic etching of Si crystal or the like and this mirror component is hybrid-integrated with the optical waveguide. Further, since the separate component is used, mass production and cost reduction are difficult.
The conventional forming method B does not satisfy the sixth subject. By the method B using inclined dicing, the inclined surface region traverses a chip, so that the inclined surface region cannot be localized. According to the conventional forming method C, the fifth subject is difficult to satisfy. By the method C using a laser beam, a degradation in optical characteristics is caused by the roughness of a laser machined surface and the thermal deformation and residual stress near the laser machined surface, so that it is difficult to ensure sufficient smoothness of the inclined surface.
According to the conventional forming method D, the third subject and the fourth subject are difficult to simultaneously satisfy, because these subjects are in a trade-off relation. In the wet etching of an isotropic material such as silica, the process proceeds concentrically from an etched surface, so that the emergent end surface of the optical waveguide over which the material having a high etching rate is not deposited by evaporation comes to lie on a concentric circle formed by the underetch under the mask, so that the shape of the end surface of the optical waveguide is curved. Accordingly, in the case that the horizontal distance from the bottom of the inclined mirror to the top of the emergent end surface is equal to the thickness of a cladding layer and that a core layer is embedded in the cladding layer at its substantially central portion, the perpendicularity of the emergent end surface becomes as low as about 60°.
Conversely, in the case of ensuring the perpendicularity of the emergent end surface, the etching quantity must be increased to obtain a large etching radius, i.e., to increase the distance between the emergent end surface and the inclined mirror. For example, in the case of obtaining an angle of 80° or more for the perpendicularity of the emergent end surface, the horizontal distance from the bottom of the inclined mirror to the top of the emergent end surface must be increased to an amount about three times the thickness of the cladding layer. Thus, the subject on the end surface perpendicularity and the subject on the end surface-to-inclined surface distance are in a trade-off relation, so that it is difficult for the conventional forming method D to simultaneously satisfy these two subjects.
The conventional forming method E does not satisfy the second subject. In the forming method by inclined RIE, second RIE for the perpendicular end surface is required, so that simultaneous formation of the perpendicular end surface and the inclined surface is difficult. Further, the shape of a groove formed by the first or inclined RIE has an acute edge (reverse mesa) obtained by subtracting the angle of the inclined surface from 90°. Accordingly, in forming a mask for the second RIE in the condition where the deep groove is present, resist cutting may occur with a high probability due to the reverse mesa edge even when a resist having a low viscosity is applied by spray coating. Further, even when a resist having a high viscosity is applied by using a general spin coater, there may arise a problem that air bubbles present in the groove cannot be removed. Thus, it is very difficult to realize the simultaneous formation of the perpendicular end surface and the inclined surface.
According to the conventional forming method F, the third subject and the fifth subject are difficult to simultaneously satisfy, because these two subjects are in a trade-off relation. In the gray-scale mask, the three-dimensional shape of the inclined surface is approximated according to the average of exposure intensities at a subpixel size forming a gray scale. Accordingly, in the case that the thickness of a photoresist film is small, sufficient approximation in the direction along the film thickness (along the depth of etching). However, in the case of a thick film required for formation of the inclined surface (the film thickness being determined by the etching selective ratio between the optical waveguide material and the photomask material), the linearity between the exposure intensity and the amount of photoreaction in the direction along the depth is degraded to cause an increase in exposure time.
Accordingly, near the top of the inclined surface where the amount of exposure is desired to be reduced, the photoreaction of the photomask proceeds in the direction along the depth, so that there is a high possibility of generation of pixel-sized unevenness. To smoothen such an uneven surface, the speed of heating after exposure and development until thermal curing of the photomask material may be increased. However, in the case of increasing the heating speed in the heating process(post-baking) after development for a general photomask material, not only a deterioration of a developed pattern due to softening occurs, but also the photomask material over the perpendicular end surface is softened to cause a deterioration in perpendicularity of the mask.
Conversely, in the case of greatly decreasing the heating speed in the post-baking process, some kind of photomask material can be cured with the perpendicularity being maintained, but the smoothness of the inclined surface cannot be obtained. Thus, in the forming method by the gray mask and RIE, it is difficult to simultaneously satisfy the subject on the perpendicularity of the end surface and the subject on the smoothness of the inclined surface because of the trade-off relation therebetween. The conclusion drawn from the above discussion is that it is difficult to realize a spatial reflection type structure which can simultaneously satisfy all of the above subjects by any one of the above conventional forming methods.